1. Field of the Invention
The present invention relates generally to air pressure drop in cooling and heating systems and more particularly to minimizing internal air resistance and increasing air flow of heat exchangers in cooling and heating systems.
2. Description of Related Art
One mature industry that is economically sensitive to costs is the heating, ventilation and air conditioning (HVAC) industry. Because of the competitive nature of both the construction and HVAC industries, HVAC systems must be inexpensive to install. Of a more global interest though, is the cost to operate and maintain HVAC systems. Often, a building owner will replace an aging HVAC system as the reduction in operating and maintenance costs can offset the retrofit cost, sometimes in a matter of months.
Broad social and energy policies also favor more efficient HVAC systems. In these days of electricity deregulation and conservation, it has become even more important to conserve energy consumption. Recently, entire electrical grids have shut down on very hot days in part because of the huge demand of HVAC systems running at extreme capacity. Furthermore, energy conservation translates directly into improved environmental conditions and decreased reliance upon foreign petroleum.
HVAC systems are typically comprised of fans and ductwork for moving air where needed. An HVAC system will include a cooling and heating section for, respectively, cooling and heating the air. In most HVAC systems, air is drawn in, filtered, cooled and dehumidified or heated and humidified, and then delivered to a room. The greatest portion of this air is drawn from the conditioned space for recirculation through the HVAC system. Considerable effort has been made to make these components more efficient.
One of several recently used methods of saving energy in an HVAC system includes the use of variable frequency drives on any motor used in a HVAC system. Variable frequency drives can also be used to selectively increase air flow. When and if the system load decreases, this can be sensed and the motors in the HVAC system will be slowed to an equilibrium value to save motor energy consumption. Another method is to reduce the design amount of outdoor air to eliminate having to condition it. Another method is an economizer cycle that utilizes 100% outdoor air when its ambient temperature is suitable for cooling the space. Another method is to replace aging equipment with newer, more efficient and more powerful equipment.
One other factor impacting design and operation of HVAC systems is indoor air quality (IAQ). One major factor in IAQ today is the amount of outdoor air introduced into an otherwise sealed space serviced by an HVAC system. The HVAC industry has adapted standards for the introduction of outdoor air into spaces serviced by an otherwise closed HVAC system. These include offices, residential, commercial, industrial and institutional spaces, as well as the interior of vehicles such as cars, buses, planes and ships. In addition to controlling indoor air for occupant comfort, the goal of HVAC systems is to provide air with reduced levels of particulates, gases and bioaerosols, be it for semiconductor, pharmaceutical or food processing facilities, hospitals, schools or offices and now the home.
Most ventilation systems today include a cooling section. The cooling section includes a type of heat exchanger typically referred to as a "cooling coil," through which air is forced and cooled. This cooling coil operates thermodynamically to remove both sensible and latent heat from the forced air. Cooling coils typically are made using aluminum fins over refrigerant tubes which have been formed into a desired shape. Essentially the same coil arrangement is used in all cooling systems, whether in HVAC systems for occupied spaces, or for refrigerators and freezers.
A similar configuration is often used in heating sections, though the thermodynamic operation is opposite to that in a cooling section. The heat exchanger of a heating section often comprises a coil. Water (or some other fluid) of an elevated temperature passes through the coil to elevate the coil's temperature. The heating coil is fashioned in a manner to promote heat transfer from the water to the heating coil. The heating coil is further fashioned to promote heat transfer from the heating coil to air which is forced across and through the heating coil.
One important measurement of a heat exchanger is its heat transfer efficiency. A heat exchanger's efficiency is essentially its ability to absorb or impart heat to an airstream. The more heat that a heat exchanger can transfer per unit of time, the greater its efficiency.
A cooling system has an efficiency defined as: ##EQU1##
A heating system has an efficiency defined as: ##EQU2##
Where, for both heating and cooling systems:
K=efficiency (coefficient of performance) PA1 Q.sub.2 =amount of heat absorbed by heat exchanger PA1 Q.sub.1 =amount of heat rejected by heat exchanger
For typical cooling systems including a cooling coil and heating systems including a heating coil, the difference between the heat absorbed Q.sub.2 and the heat rejected Q.sub.1 is the amount of work W performed.
Another important measurement of a heat exchanger is its pressure drop. A heat exchanger's pressure drop is essentially the resistance of the heat exchanger to air flowing through it. Pressure drop increases as the result of a decrease in open area, decreased open area increases the interstitial velocity between the transfer plates reducing the time the air is in contact with the transfer medium.
Under the ideal gas law (PV=nRT), the temperature of the air actually increases slightly as it passes through a heat exchanger. However, in both heating systems and cooling systems, this temperature change is inconsequential.
As a normal consequence of the process of cooling air, several things occur. One is that vapor (latent heat) is removed from the air. As moisture, it collects on the coil fins and/or anything else nearby which is below dew point, including the ductwork. Typically, a drain pan is positioned below a cooling coil. The drain pan is considered an integral part of the heat exchanger. The collected moisture runs down the coils fins and into the drain pan under the force of gravity. Water that collects in the drain pan flows away through a drainpipe equipped with a trap.
Another is that organic matter impinges and collects on the cooling coil fins from the air passing over them. Though the fins of the cooling coil appear to be smooth, in fact, when viewed under a microscope, they can be seen to have an irregular and somewhat pitted surface. The organic matter can therefore adhere easily to the damp and rough surface of the cooling coil.
Another consequence is that the cooling section is dark and at off times, it will be warm. Though when operating it will be quite cold, the cooling section will have varying cycles of cooling. When not cooling, the cooling coils typically reach room temperature.
Similar effects are encountered with heating coils, though typically to a lesser degree than with cooling coils.
Altogether, these consequences produce an environment in which molds and bacteria can grow and thrive. Over time, a heat exchanger can become near fully encrusted with microorganism activity bound to an organic substrate. The spores and products of metabolism from a heat exchanger are easily entrained into the airstream.
The drain and drain pans also become a growth environment for mold and bacteria. Water from a cooling coil may carry organic matter, including mold, spores and bacteria. The drain pans are by design points of collection for water, and the standing water and most areas in a drain pan are excellent environments for microbial growth. Organic matter and microbial activity progressively clog the drain pan's drain, exacerbating the problems and seriously impeding the primary functions of the drain pan and drain. It can be seen that the drain pan also acts as a secondary source of contamination of the cooling coil.
As the organic matter encrusts a heat exchanger, its heat exchange efficiency is compromised. The efficiency reduction does not linearly result in an energy reduction. Instead, in the case of a cooling coil, the cooling coil loses efficiency and must be made to be cooler or run longer, both of which require more energy for the same unit of work. In the case of a heating coil, the heating coil must be made to be hotter or run longer, both of which require more energy for the same unit of work. Furthermore, more energy is required to push air across the encrusted heat exchanger, resulting in an increased pressure drop. Therefore, either the fan speed must be increased, the motor horsepower increased, or both, or an oversized fan and motor are installed.
Pressure drop and heat exchange efficiency can degrade up to 30% of their original values in as little as one year, on average 22% in three years. There is an exponential decrease in heat transfer efficiency to the linear degradation of HVAC system heat exchange efficiency and airflow. There is also an exponential relationship between pressure drop and system air flow. Overall, a 30% degradation can be likened to reducing the system size by 30%.
The conventional method of controlling the accumulation and growth of substrate and microorganisms is with the use of high-pressure sprayers, surfactants, acids and biocidal agents, which are applied to all growth surfaces of the HVAC system. However, the surfactants, acids and biocidal agents are dangerous chemicals and the distribution and use of biocidal agents and acids are strictly controlled by the Environmental Protection Agency (EPA). In this age of workplace safety, there is worry not only for the occupants of the building, but also for those working on the buildings mechanical equipment. Thus, those who supply and apply these materials must use masks, gloves and gowns when handling them. These chemicals are hazardous enough that the HVAC system must be shut down and the building vacated. As can be imagined, conventional treatment can be extremely expensive.
For drain pans, in addition or as alternatives to the use of high-pressure sprayers, surfactants, acids and biocidal agents, special biocidal tablets have been used. The tablets have a relatively high cost (about $0.02/CFM). Furthermore, drain pan additives are known to react with the drain pan's protective zinc coating, eventually leading to rust. If not abated, the rust becomes a harbored habitat for microbial activity. Also, leaks will occur and can cause structural damage around the air handling unit and a building itself.
Despite the inconvenience and cost, treatment may only be effective for as little as three days to three weeks and usually not more than three months. Furthermore, chemical cleaning provides only a partial reduction of cooling coil pressure drop and a partial increase in heat exchange efficiency. To make matters worse, conventional cleaning techniques eventually damage the heat exchangers resulting in the entire heat exchanger or air handler being replaced--a very expensive event. Because of the problems with these chemicals, the continuous encrustation of heat exchangers has been largely ignored.
If done properly (i.e., regularly), the cleaning of heat exchangers can be very expensive. With cooling coils having as many as fourteen fins per inch and staggered refrigeration tubes every two inches of coil depth, cooling coils are rarely if ever cleaned completely, therefore ending in an eventual point of no return. The process is also destructive to the cooling coil, limiting the number of times the procedure can be performed.
Other, more passive solutions are also inadequate. Speeding up the fan requires new sheaves and belts. Furthermore, this results in increased energy consumption, as brake horsepower increases to the cube of fan RPM. ##EQU3##
Other solutions which have been attempted include increasing the fan motor size, speeding up the fan, replacing the fan and motor with larger ones, lowering chilled water temperature in chilled water systems, raising heated water temperature in heated water systems, and changing the time clock operation to start cooling long before building occupancy in an attempt to maintain a lower space temperature during the work day. None of these passive solutions improves the heat exchanger's efficiency but may increase it and increase the pressure drop--they only slightly compensate for the real problem. Furthermore, these passive solutions are labor and material intensive, reduce system life, often impact warranties, increase energy consumption, and result in lost work days due to system downtime and occupant discomfort.
In order to achieve minimum IAQ levels, other modifications are used. One is to introduce extra outdoor air. However, this leads to extra cooling, heating and filter costs, and may even exacerbate the heat exchanger encrustation.
Another method is to use "high efficiency particulate arrester" (HEPA) filters instead of standard particulate filters. The installation of HEPA filters, their support assemblies and maintenance is very costly. There are also very substantial indirect costs as more power from a fan is needed to push air through the denser HEPA filters, which follows the criteria indicated above. Ultimately, even HEPA filters do not solve the problem; at some point heat exchange efficiency is hindered enough to become noticeable. Adding the HEPA filter's pressure drop to an already inefficient system serves to exacerbate the problem. It will result in fewer overall air changes per hour, which reduces the amount of heat brought to the heat exchanger for absorption or rejection.
The present invention arose from testing of UVC Emitters.TM. as manufactured by Steril-Aire U.S.A., Inc., the assignee hereof. The UVC Emitters are Steril-Aire's high output germicidal lamps, which are specifically designed for cold and moving air environments such as found in HVAC systems. In the test, UVC Emitters were installed within an air handling system owned by Southern California Air Conditioning Distributors, Inc. (SCACD), in City of Industry, Calif. Specifically, UVC Emitters were installed so that their ultraviolet light output in the C band (UVC) was directed toward the cooling coil of the air handling system. The tests were unconcerned with heat transfer efficiency. Rather, these tests were designed to measure improvements to IAQ derived from eradicating mold and bacteria using the UVC Emitters.
It was clear to SCACD that the cooling coil in its air handling system was becoming less and less efficient, so that the air handling system had to consume more energy to provide its function. The cooling coil of the air handling system at SCACD's City of Industry facility was approximately twenty years old. SCACD, one of the world's largest privately owned distributors of air conditioning equipment, had been unable to prevent cooling coil encrustation in its own facility. SCACD had tried all conventional cleaning methods, which eventually provided little benefit. Thus, over time, the air handling system's heat exchanger exhibited declining efficiency. It was SCACD's expectation that the cooling coil or system would need to be replaced in order to once again obtain a reasonable amount of heat transfer.
The testing of SCACD's cooling system was performed using scientific and industry procedures under the supervision of Dr. Robert Scheir, a respected Ph.D. Prior to installation of the UVC Emitters, measurements were taken by SCACD of the air pressure drop across the cooling coil and the air entering and leaving dry and wet bulb temperatures. The UVC Emitters were then installed and the cooling coil was exposed continuously to the UVC output of the UVC Emitters for four weeks. On Sep. 28, 1997, new measurements were taken of the air pressure drop across the cooling coil and the air entering and leaving dry and wet bulb temperatures. It was concluded that the heat exchange efficiency of the cooling coil had increased and the air pressure drop across the cooling coil had decreased. SCACD's cooling coil appeared to have returned, as much as possible, to an "as new" condition, something that was heretofore believed impossible by any method. Though the UVC Emitters were believed to have some contribution to the results of the test, SCACD officials and the inventors remained skeptical that the UVC Emitters could have been exclusively responsible for the results.
It was not until several weeks later, after additional testing and analysis, that the inventors hereof were able to confidently declare that the UVC Emitters were responsible for the decreased air pressure drop and increased efficiency of SCACD's cooling coil. Furthermore, from this work, the inventors were able to formulate and refine the particular configuration, mathematics and specifications by which the heat transfer efficiency would predictably be increased and maintained in an air handling system using UVC irradiation.
The use of germicidal lamps for air sterilization only in ductwork, though once considered potentially viable, is no longer well known to those skilled in the art. Various reasons have contributed to the lack of success in utilizing germicidal lamps, except for limited and specialized purposes. The functional implementation of such devices in air moving systems has been limited generally to expensive portable units or top-of-the-wall or ceiling systems where the germicidal lamp is situated in a minimum air movement and ambient air temperature area. Germicidal lamps have sensitive physical characteristics, including plasma gases, mercury and partial pressures thereof. When germicidal lamps are used to irradiate a moving air stream, the air moving across the germicidal tube lowers the tube's temperature. The mercury condenses such that the emission of the germicidal wavelength of 253.7 nm in a conventional tube decreases as much as a 75% when the temperature falls below 58.degree. F. The phenomenon, referred to as skin-effect cooling, increases the number of conventional tubes, reduces the available square area for airflow, reduces air changes per hour, and increases the number of expensive tube replacements required to obtain an anticipated level of performance.
Germicidal lamps emit ultraviolet light at the primary and secondary emission lines of mercury (254 nm and 185 nm). At mercury's 185 nm line, ozone is created. Ozone has strict threshold limit values due to its strong oxidative properties and potential harm to humans. Despite the clear benefits of germicidal lamps, problems such as ozone, decreased output in low temperatures and moving air and the resulting short tube life have prevented their use in all but the most friendly of environments.
For further information concerning improvements in electric discharge devices which are directed to overcoming such problems, reference is made to U.S. Pat. No. 5,334,347 entitled, "Electric Discharge Device" which is co-owned with this application, and a pending application filed in the name of Forrest B. Fencl and Robert M. Culbert, entitled "Single-Ended Germicidal Lamp for HVAC Systems," application Ser. No. 08/773,463 the disclosures of which are incorporated herein by reference. Germicidal fixtures have recently become available under the Germ-O-Ray and Germitroll trademarks for installation in air ducts. The particular capabilities and design of these devices is not known to the inventors, though it is believed that both devices use conventional tubes having relatively short life and low output.